Generally, an active matrix type liquid crystal display is advantageous in many aspects, such as low power consumption, thinness, and lightness, and therefore, has been showing promise for use as a display device in diversified fields including a notebook personal computer, a mobile terminal, a TV set, etc.
Under these circumstances, there has been an increasing need for an inexpensive active matrix type liquid crystal display. To this end, various techniques have been discussed to save the manufacturing costs by improving productivity of a thin film transistor (TFT) array substrate. Among others, a technique for reducing the number of times a photomask is used during the manufacturing procedure of the active matrix type liquid crystal display has been studied extensively.
For example, Japanese Laid-open Patent Application No. 152626/1997 (Japanese Official Gazette, Tokukaihei No. 9-152626, publishing date: Jun. 10, 1997) discloses a manufacturing procedure using the photomask a fewer number of times. The following will describe an active matrix type liquid crystal display and the manufacturing method thereof in accordance with the above publication with reference to FIGS. 5(a) through 5(d), 6 and 7.
FIGS. 5(a) through 5(d) are cross sections showing the manufacturing procedure of an outlet electrode portion of a source signal line and the vicinity thereof in a TFT array substrate forming the active matrix type liquid crystal display disclosed in the above publication. Also, FIG. 6 is a plan view of the TFT array substrate forming the active matrix type liquid crystal display disclosed in the above publication. Further, FIG. 7 is a cross section explaining an arrangement of a thin film transistor 121 and the vicinity thereof in the TFT array substrate forming the active matrix type liquid crystal display disclosed in the above publication.
As shown in FIGS. 6 and 7, the active matrix type liquid crystal display disclosed in the above publication includes a TFT array substrate described more in detail below, an unillustrated counter electrode substrate provided with a counter electrode, unillustrated liquid crystal sealed in a space between these substrates, etc. The TFT array substrate has a glass substrate 101 provided with 1 a plurality of gate signal lines (scanning signal lines) 102 and a plurality of source signal lines (image signal lines) 124 formed on the surface of the glass substrate 101 through insulating films 103 and 103' in such a manner so as to intersect with each other at right angles, 2 a matrix of pixel electrodes 126 each formed at each intersection of the gate signal lines 102 and source signal lines 124, and 3 a matrix of TFTs 121 of a reverse stagger type formed in one-to-one correspondence with the pixel electrodes 126 to supply a pixel signal to the same. It should be appreciated that the TFTs 121 of the reverse stagger type do not require an etching stopper film in a channel region.
As shown in FIG. 7, each TFT 121 comprises a gate electrode G which protrudes upward perpendicularly from the gate signal line 102, a gate insulating film composed of the insulating films 103 and 103', a high-resistance semiconductor film 104 which will be made into a channel region, a low-resistance semiconductor film 105 which will be made into a source electrode S and a drain electrode D, a source metal film 106, a transparent conductive film 107, and a protection film 108, which are layered sequentially from bottom to top in this order.
Next, the following will explain a conventional manufacturing method of the active matrix type liquid crystal display with reference to FIGS. 5(a) through 5(d).
Initially, as shown in FIG. 5(a), the gate signal line 102 and gate electrode G are formed by forming a film of aluminum alloy, metal having a high melting point, or the like on the glass substrate 101 by means of sputtering, etc. and patterning (forming a pattern on) the film thus formed.
Then, as shown in FIG. 5(b), a double-layer structure composed of the insulating films 103 and 103', the high-resistance semiconductor film 104, and the low-resistance semiconductor film 105 are formed sequentially by means of plasma CVD (Chemical Vapor Deposition), etc. Subsequently, the source metal film 106 is formed on the foregoing films out of metal having a high melting point or alloy of such metals by means of sputtering, etc. Then, the source metal film 106, low-resistance semiconductor film 105, and high-resistance semiconductor film 104 thus formed are photo-etched with a pattern simultaneously by using a single photomask.
Then, as shown in FIG. 5 (c), the transparent conductive film 107 is formed on the source metal film 106 out of ITO (Indium-Thin Oxide), etc. by means of sputtering, etc. Subsequently, the transparent conductive film 107, source metal film 106, and low-resistance conductive film 105 are selectively photo-etched by using a single photomask.
Then, by forming the protection film 108 and removing a part thereof, a TFT 121', and unillustrated source signal line 124 and pixel electrode 126, etc. are formed.
Finally, as shown in FIG. 5 (d), by forming the protection film 108 out of a film of silicon nitride, etc. by means of plasma CVD, etc. and patterning the same, the protection film 108 covering the unillustrated external outlet electrode portion of the source signal line 124 and pixel electrode portion 126 is removed, while at the same time the insulating films 103 and 103' and protection film 108 covering the unillustrated external outlet electrode portion of the gate signal line 102 are removed, whereby the TFT array substrate is completed.
As has been discussed, according to the conventional manufacturing method of the active matrix type liquid crystal display, the TFT array substrate is manufactured by the manufacturing procedure which repeats the photo-litho process (photo-etching process) four times to form the pixel electrode 126, the external outlet electrode portion of the source signal lines 124, and the external outlet electrode portion of the gate signal line 102 separately.
According to the above conventional manufacturing method of the active matrix type liquid crystal display, however, the source metal film 106 is formed by means of sputtering, etc. without patterning after the insulating films 103 and 103', high-resistance semiconductor film 104, and low-resistance semiconductor film 105 are formed sequentially by means of plasma CVD, etc. Thus, an interface between the low-resistance semiconductor film 105 and source metal film 106 is quite large. In other words, both the low-resistance semiconductor film 105 and source metal film 106 are formed on the entire TFT array substrate. Thus, when the source metal film 106 is formed, these two films contact with each other in an area as large as the entire TFT array substrate. Hence, the area of the interface is substantially as large as that of the entire TFT array substrate.
Also, the semiconductor layer composed of the high-resistance semiconductor film 104 and low-resistance semiconductor film 105 has a large film stress (when a stress is applied, a corresponding strain is produced, and the film stress is defined as the ratio of stress to the strain of the film per unit area on the film surface). Thus, adhesion between the low-resistance semiconductor film 105 and source metal film 106 becomes poor due to a large interface therebetween, thereby causing problematic film separation between the low-resistance semiconductor film 105 and source metal film 106.
In short, film separation occurred during the manufacturing procedure of the TFT array substrate causes an unwanted decrease in the yield of the active matrix type liquid crystal displays.